Fine ceramic tube

In article , Andrew Mawson writes Andrew,
Does it need to be ceramic, or would glass do? If so, you should be able to source some easily. I may even have some, if I can remember where it is.
Otherwise, something from a bead shop?
David

David,
I'd thought of glass, as with a Bunsen burner I can even pull it down to the right diameter, but I reckon it may not stand up to the abrasion of hard brass wire for hours on end sawing away at it. I wondered about PTFE sleeving - comments please!
AWEM

Maybe Anderman Ceramics . For what you need maybe you could blag a sample.

Ceramic beads off an old electric fire, RS actually sell them in little bags but knowing you you will want 4 tonnes.
John S.

John - more like four grams! I actually have a box of those, but the shape is such that the brass wire would just slide off sideways as the outer profile is convex. I need parallel or concave just to be fussy. I suppose a Diablo shape would do nicely and centre the wire!
AWEM

-- Peter Fairbrother

Thanks Peter that one escaped my Googling, I've ordered a length of 2mm i/d 3mm o/d - usual thing, postage three times the goods!
AWEM

Peter,
Again many thanks - 1/2 metre of tube arrived this morning, I've sliced off the 20 mm I needed, and reinstalled the wire break switch. Mighty tough stuff to cut, ended up nicking it with a diamond file and cracking, then diamond filing to a fair finish. I'll now be ok for wire break switch replacements for several centuries
AWEM

Only just saw this, but have various sizes of ptfe sleeving if the glass tube doesn't work. Bought from J Black in N London (Stoke Newington ?) in the late 1970's and I think a job lot from some mod contractor. They were very good for coil winding and other cables as well.
What happened with the dc brake system you were building ?...
Regards,
Chris

Fine ceramic tube arrived the other day and wire beak switch back to normal operation thanks.
DC brake is working splendidly thanks. I equipped it with so many interlinks, both mechanical and electrical, to prevent the ac contactors and dc contactors coming in at the same time that it got quite complicated. Odd little quirks like the spark quenching suppressors on the DC contactors which only switch 24v DC had to be rated at 415v AC as through the low impedance DC power supply they complete a path connected to 415! - then RS Components 0.1 uF 440v AC rated capacitors come marked up '250v AC' on the side in large letters and '440v AC' in very small print on the top!!!!! (two different approval systems apparently)
AWEM

Good to hear you got it working, irrespective of how many relays. So what brake dc voltage did you use in the end and how many amps etc over what time scale to stop the motor ?. Just curious and the info might be usefull to others...
Regards,
Chris

Simple transformer / bridge rectifier / large capacitor supply giving approx 30v o/c and 24v on load. This delivers about 12 amps into the selected coil of the three phase motor which is close to the full load current of 11 amps for the 7.5 hp motor. In fact for simplicity of isolation I used three power supplies, one for each motor. Timer adjustable 0-10 seconds in fact needs about 4 seconds for a reasonable stop.
For each motor there are two contactors and two timers. Contactor #1 is the normal AC start / latch on / stop relay with thermal overload incorporated. When contactor #1 drops out, one of it's auxiliary contacts closes pulling the trigger input of timer #1 low. Timer #1 runs for approx 0.5 seconds as a safety dead band to allow contactor #1 to be fully open with definitely no 415v ac on the motor. Timer #1 drops out, triggering Timer #2 (0-10 secs) - during this Timer #2 period Contactor #2 the DC contactor closes putting 24v DC across one coil of the motor using double pole switching, and an auxiliary contact on Contactor #2 puts 240v AC on the appropriate power supply mains input. Contactors #1 and #2 are mechanically interlocked so only one can operate at a time, and also there are contacts on #1 in series with the operating coils of #2 and vice versa to absolutely assure no possibility of them being closed at the same time. Each DC contact has a suppressor across it comprising a 0.1 microfarad capacitor in series with a 100 ohm resistor. The capacitor has to be 415v ac rated as when the contacts open and the AC contactor closes one side of the DC contactor contacts is at 415v ac and you cannot assume that both dc contacts open at precisely the same instant, so with one contact open and the other closed the capacitor across the open contact is exposed to the full 415. When both open the pair are across 415 taking half each.
The dual timer is a little board sold by a chap on ebay who custom programmed it for me so one triggered the next and both had the range I wanted which I thought was a bargain for a total of £15!
AWEM